Internal combustion engines are known to convert a relatively small amount of the energy from fuel into usable energy. In a typical internal combustion engine, fuel is mixed with air, compressed within one or more combustion chambers, and ignited. The expansion of the ignited air/fuel mixture drives pistons to produce usable power. The exothermic combustion of the air/fuel mixture also generates a large amount of heat, the majority of which is lost as the high-temperature products of combustion are exhausted into the atmosphere. Exhaust heat recovery systems are known in the art as a way to convert heat that would otherwise be lost in the engine exhaust into usable energy.
While the exhaust gas from the combustion chamber represents the greatest source of recoverable waste heat, most vehicles have several other secondary sources of recoverable waste heat, although the operating temperatures of these additional heat sources are typically lower than that of the engine exhaust. Exemplary additional heat sources include engine cooling systems, oil coolers, transmission fluid coolers, exhaust gas recirculation (EGR) systems, low-temperature portions of an exhaust gas heat recovery cycle, and any other suitable source capable of providing recoverable heat.
The exhaust from the engine provides a reliable source of recoverable heat; however, the heat available from each of the secondary heat sources can vary greatly according to vehicle operating conditions and which vehicle systems are in use at a particular time. As a result, the secondary heat source that provides optimal heat recovery can change as operating conditions change.